The present invention relates to the discrimination of physical characteristics of objects. More particularly, the present invention relates to a scanning system used to detect explosives or drugs and other items in baggage using a limited view three-dimensional reconstruction.
Projection radiography has long been used for detection of metallic contraband in baggage. In general, X-rays may be used in projection radiography to measure the Compton scattering effects and photoelectric absorption to determine the number of electrons and the effective atomic number, respectively, of an object.
An example of a conventional projection radiography system is shown in FIG. 1A. The conventional projection radiography system includes an X-ray tube source 102, mounted in a suitable housing, to emit X-ray radiation toward an object 104 under inspection. A beam 106 passes through object 104 and hit a conventional X-ray screen 108 of suitable phosphorescent material.
Object 104 is supported on a conveyor belt (not shown) that moves successive portions of the object through beam 106 such that successive slices of the object are scanned by beam 106. Mounted opposite light emitting screen 108 is a photo-detector array (not shown) that may comprise a linear array of photo-diodes (not shown) positioned coextensively beneath screen 108. When X-ray photons strike screen 108, screen 108 emits light in accordance with the energy and number of X-ray photons, which depend upon the characteristics of the portion of object 104 through which the X-ray ray photons pass. The photo-diodes receive light generated by contiguous portions of screen 108, and each photo-diode generates an electrical charge in accordance with the intensity of the light received thereby.
The conventional system described above is effective in detecting materials that have a high radiographic contrast, such as metallic objects. However, organic materials that have a low radiographic contrast, such as explosives, drugs, etc., are more difficult to detect with the conventional system. Moreover, such organic materials do not necessarily have a regular shape that would otherwise aid identification.
Dual energy detection systems have been developed which can detect organic materials. In a dual energy system, two X-ray beams having characteristically different photon energies are used. Typically, organic materials tend to transmit approximately the same amount of high energy and low energy X-rays. Metals, on the other hand transmit different amount of high energy and low energy X-ray. The amount of organic material present can therefore be determined.
Other dual energy detection systems, as illustrated in FIG. 1B, use two X-ray sources 110 and 112 positioned at distinct locations around object 104. For each x-ray source, there exist a corresponding detector system 111 and 113. The fan shape x-ray projection of both first and second sources intersects at object 104. By capturing different views of the projected image of object 104, an algorithm can be used to reconstruct a three-dimensional image of the object.
In a conventional dual energy projection system, however, as well as in the single energy projection system described above, the only characteristics that can be determined are line of sight characteristics, such as the projected number of electrons and the effective atomic number along the line of sight through the object. For example, a particular high measurement of N electrons/cm2 along the line of sight could be created by either a very thin object of high density or by a relatively thick object of low density. Similarly, a measurement of an effective atomic number along the line of sight appropriate to aluminum could be caused by a plate of aluminum or by a slab of explosives coupled with a thin foil of iron. Such limited view reconstructions have serious deficiencies when called upon to detect a sheet of explosives. FIG. 1C illustrates the problem with projection imaging. A baggage scanning system has two x-ray sources 110 and 112. An enclosure 114 contains object 104 to be examined. Object 104 is, for example, a parcel or a piece of luggage containing several thin sheets of explosives 116 and 118. Unless sheet of explosives 116 is lined up to a very small angle relative to the X-ray beam as the position of x-ray source 112, its density projection spreads out so that it is overwhelmed by clutters of other objects (not shown) in object 104. Sheet explosive 116 can only be resolved by x-ray source 112. On the other hand, sheet explosive 118 cannot be resolved by either source. Thus, projection imaging alone is reliable to detect sheet explosives.
Conventional computerized tomography, such as illustrated in FIG. 1D, can overcome the problems described above associated with conventional projection radiography using both single and dual energy X-rays. That is, the three-dimensional nature of the reconstructed image generated by computerized tomography removes many of the problems associated with projection radiography and permits an absolute determination of electron densities and atomic numbers. However, conventional computerized tomography requires many views over 180 degrees of rotation about the object being scanned in order to generate a high quality reconstructed image. That is, for each cross-sectional view or slice of the object, the X-ray source must be positioned at a relatively large number of locations about the object and at each location the object is exposed and a projection (i.e., a shadow of the object) of the object is measured. Conventional computerized tomography is therefore expensive, time consuming, and requires physically large and expensive hardware. Such equipment can cost $500,000 to $1,000,000 or more per unit.
To practically implement a nationwide or worldwide system of airport baggage scanners employing three-dimensional image reconstruction to more reliably detect contraband, weapons and dangerous materials requires a less expensive and more physically compact approach. Many units will be required to process streams of baggage quickly enough so that flights are not unduly delayed by the system. The units need to be reasonably affordable to the airlines that will buy them and the equipment must reasonably fit in available space in existing airports.
A method and apparatus for determining a specified physical characteristic in an object exposes the object to at least two angularly fixed sources of electromagnetic radiation to create projected images of the object. The sources are rotationally scanned about the object so as to oscillate in an angular range about the object. The sources are relatively stationary to one another. A three dimensional reconstructed image of the object based on the projected images of the object is created and examined for a specified physical characteristic in the object.